Studies of spouse similarity for alcohol and tobacco consumption find correlations that are moderate to strong in magnitude. The assessment of assortative mating for alcohol- and tobacco-related behaviors is critical for obtaining unbiased estimates of genetic and shared environmental effects in quantitative behavioral genetic studies; however, assortment effects have rarely been taken into account. Furthermore, many earlier studies of assortment lacked comprehensive testing of alternative explanations of spouse similarity to that of phenotypic assortment, including social homogamy. We propose to use a genetically informative twin-kinship design, including monozygotic and dizygotic twins, their spouses, and offspring, to resolve the mechanisms of assortment as well as models of parent-offspring transmission. The study will focus on data collected in 1977 from a population-based sample of 138 Swedish twin pairs and their spouses born between 1911 and 1935. The twin-kinship sample will be augmented with 745 same-sex twin pairs from the Swedish Adoption/Twin Study of Aging (SATSA). Assessments of tobacco and alcohol use were made by self-report questionnaire surveys. Information on other substance use (e.g., caffeine, tranquilizers) and socioeconomic indicators was also gathered. Behavioral genetic analyses will be utilized to address the following six questions: (1) What mechanisms of assortment account for spouse similarity for alcohol and smoking consumption? (2) What is the impact of assortment on estimates of genetic and environmental contributions to individual differences on alcohol and tobacco consumption under various models? (3) Is the assortment for alcohol and tobacco consumption primary or does it occur secondarily to assortment for education or other socioeconomic factors? (4) Is there a more complex phenotype of substance use on which assortment takes place? (5) What model of parent-offspring transmission accounts for familial similarity given an appropriate model of assortment? (6) To what relative extent do common genetic and environmental influences account for the bivariate relationship between alcohol and tobacco consumption? This study will also investigate potential generation and gender differences.